The present invention relates generally to the drying of fibrous sheet materials by transmitting heated air through such sheet material with a rotary through dryer apparatus, and in particular to such an apparatus in which the through dryer drum contains a plurality of separate independently controlled vacuum chambers of different pressure circumferentially spaced about the axis of the drum and aligned with separate sources of hot air outside the drum which also may be independently controlled to provide different air temperatures.
The through dryer apparatus of the present invention is especially useful for the manufacture of porous paper products, such as tissue and towels. However, it is also possible that the through dryer apparatus of the present invention can be used for drying other porous fiber sheet material.
Through dryers have previously been used for drying paper, as shown in U.S. Pat. No. 3,432,936 of R. I. Cole et al. However, most conventional paper making machines employ one or more heated nonforaminous dryer drums called Yankee dryers, which dry the paper by contact with the surface of such drum. Yankee dryer drums are usually heated by supplying steam to the interior of such drum, which requires an extremely complex, expensive system for distributing steam to each dryer drum. In addition, such conventional drying systems are inefficient and for many types of paper require a large number of drying drums so that the dryer section of a typical paper machine extends over a long distance. The prior patented apparatus is inefficient and requires both a through dryer and a Yankee dryer as well as felted press rolls for removing the water from the paper after such paper is transferred to a second wire from the fourdrinier wire on which it is formed.
In contrast, the drying apparatus of the present invention employs the through dryer drum directly on the forming wire for drying the paper on the same wire where the paper is formed from pulp applied to such wire. The result is a more efficient drying apparatus which uses less energy to perform the drying operation. This extremely fast, efficient drying operation is achieved by providing a through dryer drum with a plurality of internal vacuum chambers having separate exhausts connected to different vacuum means for independently controlling the vacuum pressure and air flow in such chambers, and by employing separate sources of hot air of different independently controlled temperatures above such vacuum chambers. The term "air" as used in this application includes all types of gas suitable for drying purposes.
The above-mentioned patent discloses two exhaust conduits for two vacuum chambers having a common heated air source which extends through the hollow support shaft on which the drum is mounted, thereby limiting the size of the exhaust conduits, which reduces the maximum air flow as well as limiting the maximum air temperature to below that which will damage the shaft bearings. These problems are avoided in the present invention by using external trunnion rollers for mounting the through dryer drum to provide completely open space within the drum for the vacuum chambers. The trunnion rollers do not limit the size or position of the exhaust conduits for such chambers. As a result the exhaust conduits can be larger in diameter for greater air flow and extend through the end of the drum at positions spaced from the trunnion roller bearings to enable higher exhaust air temperatures.
Through dryers have been previously provided with separate external dryer hoods forming heating zones of different temperature spaced around the periphery of the dryer drum, as shown in U.S. Pat. No. 3,541,697 of Villalobos, U.S. Pat. No. 3,098,371 of Fleissner, and U.S. Pat. No. 3,891,500 of Kankaanpaa. However, in all cases these patents have employed a single vacuum chamber within the dryer drum and therefore cannot provide different independently controlled vacuum chambers beneath each of the hot air supply hoods, in the manner of the present invention. Furthermore, non of these through dryers are mounted on external trunnion rollers, as in this invention, to enable a plurality of vacuum chambers to be provided within the dryer drum with a plurality of separate exhaust conduits extending through the end of such drum which are independent from, and not limited in size or position by the drum support means.
Nonforaminous dryer drums, such as those used in Yankee dryers, and press rolls have been supported on trunnion rollers, as shown in U.S. Pat. No. 2,878,583 of Spooner, and U.S. Pat. No. 3,087,538 of Newman. However, unlike the present invention, this was not done to enable a plurality of vacuum chambers and their separate exhaust conduits to be mounted within the drum.
Previous attempts to use through dryers on fourdrinier wires without further drying on another wire, such as shown in U.S. Pat. No. 2,753,766 of Simpson, have been unsuccessful, among other reasons because the through dryers employed a single heating zone and a single vacuum chamber rather than a plurality of independently controlled vacuum chambers and independently controlled heating zones, in the manner of the present invention. As a result, such paper machines using through dryers on the forming wire have required additional drying after transferring the paper to another wire, as shown in U.S. Pat. No. 3,447,247 of Daane.